Method and device for producing molten material

ABSTRACT

A method for producing molten material, wherein oxygen, reducing agents and iron that has been reduced in a reduction reactor are introduced into a melter gasifier. The reducing agent is gasified with the oxygen and the heat thereby produced melts the reduced iron. Cupola gas from the melter gasifier is used at least as a portion of the reduction gas, and reacted top gas is withdrawn from the reduction reactor. The aim of the invention is to increase energy efficiency and raw material efficiency as well as productivity while at the same time obtaining metallurgically improved properties of the product. For this purpose, at least a portion of the top gas is branched off from the line for the withdrawal of the top gas from the reduction reactor and is returned via at least one return line leading to the melter gasifier and is introduced into the melter gasifier.

The invention relates to a method for production of molten metal, oxygen, reducing agent and iron reduced in a reduction reactor being introduced into a melt gasifier, the reducing agent being gasified with the oxygen, and the reduced iron being melted by means of the heat which in this case occurs, the cupola gas from the melt gasifier being used as at least a fraction of the reduction gas, and reacted top gas being drawn off from the reduction reactor, and also to a plant for carrying out the method, with a reduction reactor, with a melt gasifier having an oxygen supply and with a supply system for reducing agent, at least one line for supplying the cupola gas from the melt gasifier into the reduction reactor and at least one line for drawing off the top gas from the reduction reactor.

In blast furnaces, various carbon-containing gases, such as natural gas, coke oven gas, etc., are injected via the tuyers or in the bosh plane, with the aim of saving coke and increasing profitability, as already described, for example, in GB 883 998 A. An injection of blast furnace gas is not economical because of the high CO₂, N₂ and low H₂ content.

In melt-reduction plants, as described, for example, in DE 36 28 102 A1, oxygen with a temperature of 25° C. and a purity of ≧95% by volume is injected via nozzles into the melt gasifier, in order to gasify the reducing agents (predominantly coal and coal briquets) and make available the heat required for melting the reduced iron. The cupola gas of the melt gasifier (ESV) is used for indirect reduction in a fixed-bed reduction shaft (FBRS) or in fluidized-bed reactors (WSR). Owing to the lack of utilization of gas in the FBRS or WSR, a high specific coal or coal briquet consumption and a high energy excess in the export gas are obtained.

Coupling the operation of the melt gasifier with the reduction reactor affords a fluctuating metallization of the iron slurry of 70-90%.

For example, a rise in the char-bed and cupola temperature in the melt gasifier leads to a reduced required oxygen quantity, therefore also to a decrease in the reduction gas. As a result of this decrease, the metallization in the fixed-bed reduction shaft or fluidized-bed reactor also falls, thus, in turn, causing a drop in the char-bed and cupola temperature in the melt gasifier. However, this leads to a higher oxygen requirement, and therefore the quantity of reduction gas rises and metallization increases again. Owing to the long control system, therefore, it is not possible to have a stable operation of the melt gasifier (due, inter alia, to coal breakdown), thus resulting in higher specific reducing agent consumptions.

Further, the adiabatic flame temperature (RAFT) occurring during the gasification of the coal with oxygen lies above 3000° C. (theoretic), with the result that the reduction of the SiO₂ into Si is promoted and therefore the pig iron may have high silicon contents. Consequently, additional retreatment is often necessary in order to achieve the desired Si values of 0.4-0.5% by weight.

The purified export gas, which is composed of the blast furnace gas from the direct reduction assembly and of the cupola gas from the melt gasifier, has the following typical analysis at 1.5 barg: CO 45% by volume, CO₂ 30% by volume, H₂ 19% by volume, H₂O 3% by volume and N₂ 3% by volume. Owing to the gas excess, it has to be delivered for utilization and overall energy optimization.

The object of the present invention, therefore, was to specify a method and a plant, as initially described, in which, along with an increased energy and raw material efficiency, productivity can also be increased, while at the same time metallurgically better properties of the product are obtained.

To achieve this object, according to the invention, the method is characterized in that at least part of the drawn-off top gas is introduced into the melt gasifier. As a result of this injection, significant savings of coal and coal briquets as reducing agents in the melt gasifier are possible, these being replaced by the supply of reductants (CO, H₂) from the recirculation gas. Moreover, a cooling of the raceway and of the char-bed is achieved by a directed lowering of the flame temperature which is obtained by virtue of the endothermal reaction of the coal, coal briquets or coke with the gas constituents and the cracking of the methane.

Advantageously, in this case, the recirculated gas is compressed.

According to a further advantageous variant of the method, there is provision for the recirculated gas to be cooled, between compression and introduction into the melt gasifier, preferably to 30 to 50° C., and for the carbon dioxide content to be reduced, preferably to 2 to 3% by volume. The advantage of this is a higher gas quantity in the char-bed for indirect gas reduction, that is to say more reduction work performed in the melt gasifier.

If, according to a further variant, at least part of the recirculated gas is only compressed, at least a further part of the recirculated gas is only cooled and its carbon dioxide content reduced, and the compressed gas and the carbon dioxide-reduced gas are mixed before introduction into the melt gasifier, the influencing of the properties in the melt gasifier can be metered even more accurately.

For this purpose, there may also be provision for the recirculated and at most cooled and carbon dioxide-reduced gas to be heated before introduction into the melt gasifier, preferably using a part stream of the recirculated gas as fuel gas. By the recirculation gas being preheated, the recirculatable gas quantity can be maximized, without the adiabatic flame temperature (RAFT) falling below an undesirably low limit, with disadvantages for metallurgy. This results in an additional advantageous reduction of the use of raw materials and an additional possibility for monitoring the process.

According to a method variant according to the invention, there may be provision for at least one part stream of the recirculated gas to be reformed with higher hydrocarbons, using a further part stream of the recirculated gas as fuel gas.

In this case, advantageously, the reformed recirculated gas can be mixed with the only compressed and/or the cooled and carbon dioxide-reduced gas before introduction into the melt gasifier.

According to an advantageous method variant, there is provision, further, for particles cotransported in the cupola gas to be separated and recirculated into the melt gasifier, a part stream of the only compressed and/or of the cooled and carbon dioxide-reduced gas being admixed for the transport of the recirculated particles.

According to a method variant according to the invention, there may be provision for the theoretical adiabatic flame temperature in the raceway to be controlled by means of the quantity and/or temperature and/or CO₂ fraction of the recirculated gas, with the result that a directed control of the metallurgical processes becomes possible.

As a result of each individual possibility of action of those described, but also due to combinations of these, an efficient control of the theoretic adiabatic flame temperature in the raceway is possible.

The plant described initially is characterized, according to the invention, in order to achieve the object, by at least one return line branching off from the line for the top gas and leading into the melt gasifier.

In order in this case to minimize the risk of fire or explosion, the return line for the gas runs parallel to the oxygen supply as far as the issue of the latter.

Advantageously, a compressor is inserted into the return line.

An advantageous embodiment of the plant is characterized, according to the invention, in that, a cooling device and a carbon dioxide reduction stage are inserted between the compressor and the oxygen supply, the latter also being capable of reducing or completely eliminating the steam content.

In this case, there may be provision for the outlet of the compressor and the outlet of the carbon dioxide reduction stage to lead into a common supply line to the oxygen supply to the melt gasifier.

So that the recirculatable gas quantity can be maximized by the preheating of the recirculation gas, without disadvantages for metallurgy on account of an excessive lowering of the adiabatic flame temperature (RAFT), a heating device is provided downstream of the convergence of the outlet of the compressor and of the outlet of the carbon dioxide reduction stage. This affords an additional advantageous reduction in the use of raw materials and an additional possibility for monitoring the process.

Owing to the advantageous further feature of the invention that the heating device operates with fuel gas, a branch emanating from the return line upstream of the compressor and leading to the fuel gas connection of the heating device, the use of raw materials can be reduced and consequently the efficiency of the plant can be further increased.

Advantageously, a reformer may be inserted between the compressor and the oxygen supply.

In this case, too, the consumption of raw materials can be reduced, in that, according to an advantageous embodiment, a branch emanates from the return line and leads to a fuel gas connection of the reformer.

A further embodiment of the plant according to the invention is characterized in that, a cooling device and a carbon dioxide reduction stage and also a reformer are provided in parallel branches of the return line, said parallel branches leading into a common supply line to the oxygen supply to the melt gasifier.

Preferably, in at least one line for the cupola gas, a particle separator is provided, from the particle discharge of which a particle recirculation leads to the melt gasifier, a branch from the return line issuing into the particle recirculation.

The invention will be explained in more detail in the following description by means of a preferred exemplary embodiment and with reference to the accompanying drawing.

Particulate or pellet-shaped iron ore is fed, if appropriate together with unburnt aggregates, into a reduction shaft 1. Iron slurry generated in the reduction shaft 1 is introduced via discharge devices 2 into the head of a melt gasifier 3. At the bottom of the melt gasifier 3, liquid pig iron collects, and, above this, liquid slag, which in each case are drawn off preferably discontinuously via specific taps. The melt gasifier 3 is supplied from a storage shaft 4 with a gasification agent, preferably coal and/or coal briquets, in any event mixed with screened-out undersize of the iron ore which could not otherwise be used for the reduction process. An oxygen-containing gas is supplied via gas lines 5 in the lower region of the melt gasifier 3.

The reduction gas generated is led out of the head of the melt gasifier 3 via a line 6, freed in a hot-gas cyclone 7 of solid constituents, in particular dust coal and fine-grained degassed coal, and then passes via a line 8 into the reduction shaft 1. In the latter, the reduction gas flows through the column of iron ore and aggregates in countercurrent and at the same time reduces the iron ore into iron slurry.

The degassed coal dust separated in the hot-gas cyclone 7 and other particulate contents are recirculated to the melt gasifier 3, preferably being gasified on entry into the latter through dust burners which are arranged in the wall of the melt gasifier 3 and to which oxygen-containing gas is also delivered.

The at least partially consumed reduction gas is drawn off at the upper end of the reduction shaft 1 via a top gas line 9 and, after scrubbing in the wet scrubber 10, is delivered as export gas for utilization and overall energy optimization on account of the gas excess. Reduction gas used for regulating the pressure of the plant is, after scrubbing in the wet scrubber 11, either admixed to the export gas or recirculated via the line 12 as cooling gas into the line 6 upstream of the hot-gas cyclone 7.

It is particularly advantageous to utilize at least part of the drawn-off top gas or, after scrubbing, of the export gas by recirculation into the process itself, to be precise by recirculation and introduction into the melt gasifier 3. For this purpose, the top gas to be recirculated is branched off, downstream of the wet scrubber 10, via a line 13 and compressed by means of a compressor 14 with as high a suction pressure as possible. Advantageously, reduction gas not required may also be branched off and recirculated, downstream of the wet scrubber 11, via a further line 15, even before admixing to the export gas.

According to a first variant, after intermediate cooling to 30-50° C. in the cooler 16 and reduction of the CO₂ content to 2-3% by volume in the plant 17, the recirculated top gas can be injected into the melt gasifier 3 for the removal of CO₂ via lances 18 which are introduced into the oxygen nozzles, the return line for the top gas running as far as the issue of the oxygen supply and parallel to the latter. Part of this gas treated in this way can be branched off and admixed, for transport, to the particles recirculated from the hot-gas cyclone 7. In addition to the saving of coal and coal briquets as reducing agents in the melt gasifier by the supply of reductants, such as, for example, CO or H₂, from the recirculated top gas, a cooling of the raceway and of the char-bed can also be achieved due to the directed lowering of the flame temperature on account of the endothermal reaction of the coal, coal briquets or coke with the gas constituents and the cracking of the methane, the following reactions being critical:

C+CO₂→2 CO ΔH₂₉₈=+173 kJ/mol

C+H₂O→CO+H₂ ΔH₂₉₈+132 kJ/mol

CH₄→2 H₂+C ΔH₂₉₈+74 kJ/mol

The installation of the compressor 14 and, if appropriate, of the CO₂ removal plant 17 with a preceding heat exchanger 16 or of a reformer/reduction gas furnace 21 also affords the advantages that higher melting performances and therefore an increase in productivity are possible, that, by reduced use of reducing agents, a reduction in the specific CO₂ emissions per ton of pig iron can also be achieved, and that a lowering of the operating costs and therefore the rapid payback of the additional investment costs, depending on the reducing agent cost for coal, coal briquets and coke, are possible. Even use as a nitrogen replacement in dust burners could be envisaged.

In any event, the top gas may also be introduced directly, utilizing the sensible compression heat. To regulate the CO₂ content, for example as a function of the char-bed or cupola temperature, the two gas streams may also be mixed.

The recirculated top gas may also optionally be heated, after CO₂ removal, by means of a reduction gas furnace 19 (convective, regenerative), electrical heating, plasma burners or heat exchangers (utilization of the sensible heat of process gas, for example top gas upstream of the scrubber), etc. In this case, if a reduction gas heating furnace 19 is used, part of the branched-off top gas is employed via the line 20 as fuel gas.

In heating the recirculated top gas by a heat exchanger before introduction into the melt gasifier 3, the heat energy of the top gas upstream of the wet scrubber 10 is preferably utilized. This affords the advantage of increasing the energy efficiency of the process due to smaller process water quantities required for cooling the top gas, which also means a reduction in the energy demand of the process water pumps. Further, there is a reduction in the heat which is discharged from the top gas into the process water and which is lost via cooling towers or by evaporation causes water losses in the system which constantly have to be compensated.

Alternatively, the recirculated top gas may also be reformed with higher hydrocarbons (for example, natural gas) in a reformer 21, part of the top gas supplied via a line 22 as fuel gas being used for the endothermal reaction heat.

The quantity of reduction gas from the melt gasifier 3 which is increased due to gas recirculation is utilized for increasing production in the reduction stage 1 (shaft or fluidized bed) and/or for constant metallization. Constant metallization is achieved by the decoupling of the melt gasifier 3 and the reduction shaft 1. The quantity of reduction gas which is sufficient at all times allows constant metallization in the reduction shaft 1. There is consequently no need for any major changes in the oxygen quantity to be supplied to the melt gasifier 3 in order to adapt the thermal economy, thus leading to a constant char-bed temperature, lower coal breakdown and therefore a stable operation of the melt gasifier 3 along with low specific reducing agent consumption. Optimization of the melt gasifier operation leads to a smaller necessary quantity of reducing agents for the fixed-bed reduction shaft 1 (FBRS) or in fluidized-bed reactors (WSR) of the plant, this necessary quantity being entirely compensated by the recirculation of top gas.

Furthermore, this results in the possibility of rapid regulation, a lowering of the silicon content in the pig iron due to a lower adiabatic flame temperature and a more stable operation of the melt gasifier, in order to minimize the silicon reduction taking place at high temperatures, according to the following formula:

SiO₂+2 C→Si+2 CO ΔH₂₉₈=+690 kJ/mol

In addition to the silicon content, a reduction in the sulfur content in the pig iron can also be achieved, since, owing to the recirculation of the top gas with only 1 to 100 ppm of H₂S, a substantially lower introduction of sulfur occurs than during the sole use of coal, coal briquets or coke.

Finally, by gas recirculation, the setting of the necessary nozzle velocity and of a sufficient penetration of the raceway, along with lower melting rates, is appreciably facilitated. 

1. A method for production of molten metal, comprising reducing oxygen, reducing agent and iron in a reduction reactor and introducing them into a melt gasifier, wherein the reducing agent is gasified with the oxygen, and the reduced iron is melted by the heat which in this case occurs, using cupola gas present in the melt gasifier as at least a fraction of the reduction gas, and drawing off reacted top gas from the reduction reactor, introducing at least part of the drawn-off top gas into the melt gasifier and compressing the gas recirculated to the melt gasifier, at least one of cooling the recirculated gas between its compression and its introduction into the melt gasifier, reducing the carbon dioxide content of the recirculated gas, reforming at least one part stream of the recirculated gas with higher hydrocarbons, using a further part stream of the recirculated gas as fuel gas, and controlling the theoretical adiabatic flame temperature in a swirl zone by at least one of quantity, temperature and CO₂ fraction of the recirculated gas.
 2. The method as claimed in claim 1, wherein at least a first part of the recirculated gas is only compressed, at least a second part of the recirculated gas is only cooled and the carbon dioxide content thereof is reduced, and mixing the compressed gas and the carbon dioxide-reduced gas before their introduction into the melt gasifier.
 3. The method as claimed in claim 1, further comprising heating the recirculated and at most cooled and carbon dioxide-reduced gas before introducing them into the melt gasifier, and using a part stream of the recirculated gas as fuel gas.
 4. The method as claimed in claim 1, further comprising mixing the reformed recirculated gas with at least one of the only compressed and the cooled and carbon dioxide-reduced gas and then introducing the mixed gas into the melt gasifier.
 5. The method as claimed in claim 1, further comprising separating particles cotransported in the cupola gas and recirculating the particles into the melt gasifier, and admixing a part stream of at least one of the only compressed and of the cooled and carbon dioxide-reduced gas for transporting the recirculated particles.
 6. The method as claimed in claim 1, further comprising cooling the recirculated gas, between its compression and introduction into the melt gasifier, to 30 to 50° C.
 7. The method as claimed in claim 1, wherein the carbon dioxide content is reduced to 2 to 3% by volume.
 8. A plant for the production of molten metal, comprising a reduction reactor, a melt gasifier having an oxygen supply and a supply system for reducing agent, at least one line for supplying cupola gas present in the melt gasifier into the reduction reactor, and at least one first line for drawing off the top gas from the reduction reactor, at least one return line branching off from the first line for the top gas and leading into the melt gasifier; a compressor inserted into the return line; a cooling device and a carbon dioxide reduction stage inserted between the compressor and the oxygen supply, a reformer inserted between the compressor and the oxygen supply, the cooling device, the carbon dioxide reduction stage and the reformer are provided in parallel branches of the return line; and a common supply line to the oxygen supply to the melt gasifier, and the parallel branches leading into the common supply line.
 9. The plant as claimed in claim 8, wherein the return line for the gas runs parallel to the oxygen supply as far as the issue of the latter.
 10. The plant as claimed in claim 8, wherein the compressor has an outlet and the carbon dioxide reduction stage has an outlet and those outlets lead into a common supply line to the oxygen supply to the melt gasifier.
 11. The plant as claimed in claim 10, further comprising a heating device downstream of the convergence of the outlet of the compressor and of the outlet of the carbon dioxide reduction stage.
 12. The plant as claimed in claim 11, wherein the heating device operates with; a branch emanating from the return line upstream or downstream of the compressor and leading to the fuel gas connection of the heating device.
 13. The plant as claimed in claim 8, further comprising a branch emanating from the return line and leading to a fuel gas connection of the reformer.
 14. The plant as claimed in claim 8, further comprising at least one line for the cupola gas, a particle separator having a particle discharge with a particle recirculation leading to the melt gasifier, and a branch from the return line issuing into the particle recirculation. 